Spectral signal quality of fast pressure sensitive paint measurements in turbulent shock-wave/boundary layer interactions

This study presents a critical investigation of the signal quality obtained using candidate fast pressure sensitive paint measurements in realistic, spectrally complex flows. The evaluations focus on characterizing the importance of the paint layer frequency response in conjunction with the fundamental signal-to-noise ratio limitations of the imaging system. Fast fluoro-isopropyl-butyl polymer-based paint is used primarily because of an identified characteristic attenuation pattern. Complementary high-speed pressure sensitive paint and wall static pressure measurements are performed in a turbulent, separated, scooped compression ramp shock-wave/boundary layer interaction at Mach 2.5 using a sampling rate of 4 kHz. The results reveal a systematic underprediction of the RMS pressure due to the limited frequency response of the paint. A simple frequency domain correction methodology utilizing the experimental data and a diffusion-based model of the paint response are employed to compensate for these effects. A method for determining the spectral signal-to-noise ratio of the imaging system is then presented, which is found to impose a major constraint on the upper limit of the resolvable pressure frequency. The correction is observed to provide a significant improvement in the agreement between the pressure sensitive paint and transducers, but only below the noise-dominated cutoff frequency. The relative significance of these factors is then investigated for polymer–ceramic-based, 8 kHz measurements of a planar compression ramp shock-wave/boundary layer interaction in a rectangular channel.